Smooth textured wet-laid absorbent structure

ABSTRACT

A smooth-textured non-woven, superabsorbent particle-impregnated fibrous structure is disclosed. The web exhibits an improved smooth surface texture. The structure contains from 50% to 80% of ion sensitive SAP having a particle size of less than 200 microns, wood pulp fibers and cellulose acetate fibers, each in preferred specified amounts indicated herein. The superabsorbent, ion sensitive polymer particle-impregnated fibrous structure is made from an aqueous, wet-lay process in which an aqueous furnish comprises solids of fibers and superabsorbent, ion sensitive polymer and dissolved salt, such as preferrably, Na 2  SO 4 . The furnish is passed over a moving foraminous support, such as a Fourdrinier wire, and a wet web structure is formed, followed by drying.

FIELD OF THE INVENTION

This invention relates to a wet-laid nonwoven structure containingfibers and water insoluble, water-swellable, superabsorbent, ionsensitive polymer particles (SAP) and process for making an absorbentstructure. The structure is intended for use in absorbent hygieneproducts such as diapers, incontinence pads, sanitary napkins andtampons and in wiping materials for mopping up spills of fluids. Awet-laid nonwoven fabric is a fabric comprising fibers which have beendeposited from an aqueous suspension onto a moving foraminous support.

BACKGROUND OF THE INVENTION

Fibrous, non-woven, superabsorbent, ion sensitive polymer-impregnatedstructures are known.

See generally, U.S. Pat. Nos. 5,167,764, 5,607,550, 5,516,585 andEuropean Publication No. 437,816. Additionally, the following referencesdisclose previously attempted methods of handling superabsorbent orhydrogel ion sensitive polymers to obtain superabsorbent structures. SeeU.S. Pat. Nos. 3,669,103; 4,610,678; 4,986,882; 5,049,235; 5,137,600;5,160,789; 5,443,899; 5,531,728; and 5,547,745. See U.S. Pat. No.5,516,585 teaching the well known binder polymers suggested for use withair laid nonwoven structures containing SAP.

EP-A-437816 discloses a nonwoven wet-laid superabsorbent materialproduced by the process of blending superabsorbent, ion sensitivepolymer particles with a liquid to form a slurry, mixing particles withthat slurry, filtering that slurry/fibre mixture to remove a portion ofthe liquid and drying the superabsorbent slurry/fiber mixture to form anonwoven wet-laid superabsorbent material.

EP-A-359615 discloses a method for the manufacture of a superabsorbentfibrous structure in which a dry solid absorbent is applied directly toa wet-laid web of cellulosic fibers prior to drying the wet web.

EP-A-273075 discloses a high water-absorbency paper made by sheeting amixture of wood pulp fiber, water-soluble resin and highwater-absorbency resin.

A number of techniques for applying binders to webs of fibers are known.For example, U.S. Pat. No.4,600,462 of Watt describes a process in whichan adhesive binder is sprayed onto one or both surfaces of an air laidcellulose fiber web. Submersion of the web in the adhesive binder isanother method disclosed in this patent of applying the binder.Individual binder coated fibers for mixing with other fibers are notproduced by this process. A hydrophile solution is also applied to theweb. As another example, U.S. Pat. Nos. 4,425,126 and 4,129,132 ofButterworth, et al. describe a fiberous material formed by combiningthermoplastic fibers and wood pulp, heat fusing the combined fibers, andthereafter depositing a binder on the heat fused web. Because the fibersare heat fused prior to adding the binder, individual binder coatedfibers for mixing with other fibers are not produced by this process.

Absorbent products such as diapers which include particles of asuperabsorbent ion sensitive polymer such as crosslinked sodiumpolyacrylate disposed between layers of wood pulp are known for examplefrom EP-A-257951.

The use of fibers of water-swellable water-insoluble superabsorbent, ionsensitive polymer is disclosed in U.S. Pat. No. 5,607,550, wherein it istaught that incorporation of superabsorbent, ion sensitive polymers inparticulate form in the fiber web has significant disadvantages in manyrespects. The prior art teaches that superabsorbent, ion sensitivepolymer particles are less securely retained both during formation ofthe wet-laid nonwoven structure and when the structure is in furtherprocessed during incorporation into an absorbent product. Moreover theart indicates a relatively less uniform dispersion of superabsorbent,ion sensitive polymer particles in the web occurs as opposed to thedispersion of the SAP fiber in the web. It is also taught conventionallythat with superabsorbent, ion sensitive polymer particle-impregnatedstructures, the particles become loosely attached to the fibrousstructure of the nonwoven fabric.

In order to provide sufficient absorbency performance necessaary forutilization in an absorbent article, it has been found that the loadingof superabsorbent in a web must be at least about 50% by weight of theentire structure. However, significant loadings of particles in thefiber structure (such as above about 50% SAP particles on the totalweight of the web) require sufficient strength of the wet web in theprocess

Whereas the cost associated with forming fibers of superabsorbent, ionsensitive polymer is inherently higher than that of the particulate SAP,it would be desirable to overcome the aforementioned drawbacks in theuse of particles of SAP. Composite structures of fibers impregnated withsuperabsorbent, ion sensitive polymer particles could greatly reduce themanufacturing cost of end use products such as those aforementioned,however when forming such a structure with 50% or higher loadings ofparticles of SAP, the web thus formed has a rough texture which can befelt in a disposable article having a top sheet. The texture istelegraphed through the article and may provide undesirable comfort forthe user.

Accordingly, there is an unmet need to develop improved superabsorbent,ion sensitive polymer particle-impregnated structures having improvedtexture for their intended uses. An improved superabsorbent, ionsensitive polymer particle-impregnated structure has been isolated andfound to exhibit unexpected smooth texture and surprisingly goodabsorbency.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a non-woven,wet-laid, superabsorbent, ion sensitive polymer particle-impregnatedfibrous structure having smooth surface texture and which is free ofbinder polymer. All percentages specified herein are weight percentages.Specifically, the structure comprises from 50% to 80% of ion sensitiveSAP having a particle size of less than 200 microns, wood pulp fibersand cellulose acetate fibers, each in preferred specified amountsindicated below. The superabsorbent, ion sensitive polymerparticle-impregnated fibrous structure is made from a aqueous, wet-layprocess in which an aqueous furnish comprises solids of fibers andsuperabsorbent, ion sensitive polymer. The aqueous furnish contains 0.2%to 5% or more dissolved salt, which is preferredly Na₂ SO₄. The furnishis passed over a moving foraminous support, such as a Fourdrinier wire,and a wet web structure is formed. The wet web structure is conveyed toan in-line washing zone with a sufficient volume of water, preferablyimpinging the wet web in a continuous curtain flow, and the washed wetweb structure is dried to form the non-woven, wet-laid, superabsorbent,ion sensitive polymer particle-impregnated structure (web).

The wet-laid, superabsorbent, ion sensitive polymer particle-impregnatedstructure, on a dry weight basis, comprises about 50% to about 80% waterinsoluble, water swellable ion sensitive polymer (SAP) and about 20% toabout 50% fibers (fibrous portion). The fibrous portion of the webcomprises 5% to 50% cellulose acetate fibers and 50% to 95% pulp fibers.Preferably the fibrous portion comprises 10% to 50% cellulose acetatefibers and 50% to 90% wood pulp fibers. More preferredly the fiberrousportion comprises 10% to 40% cellulose acetate fibers and 60% to 90%wood pulp fibers. Most preferredly, the fibrous portion comprises 5% to20% cellulose acetate fibers and 80% to 95% wood pulp fibers.

The wet-laid, superabsorbent, ion sensitive polymer particle-impregnatedstructure contains from 0% to 35% residual salt in the dried web,preferably from 5% to 35% such residual salt remains, and mostpreferably 10% to 16% residual salt. The level of residual salt on thetotal weight of the dried web has been found to correlate inversely withthe AUL absorbency performance, described below. The minimum AULabsorbency of the wet-laid, superabsorbent, ion sensitive polymerparticle-impregnated web which contains from 5% to 35% residual salt isgreater than or equal to about 13 g/g. AUL on the basis of SAP polymerin the sheet is greater than or equal to 19 g/g within the range of 5%to 35% salt. The AUL on the basis of the SAP in the sheet is obtained bydividing the AUL of the web by the percent SAP in the web. Preferably ata lower residual salt content less or equal to about 20%, AUL absorbencyper unit SAP in the web is greater than or equal to about 23 g/g. Mostpreferredly the AUL absorbency per unit of SAP in the web is greaterthan or equal to 30 g/g and the salt content in the web is between about10% and 16%.

DETAILED DESCRIPTION OF THE INVENTION

The SAP/fiber web is produced by the process referred to as wet layprocessing. An exemplary apparatus known in the art is an inclined wireforming machine, which can be used to form the wet-laid, superabsorbent,ion sensitive polymer particle-impregnated web. The web is transferredfrom the inclined wire to a horizontal conveyor optionally equipped withvacuum suction ports to further remove processing water. A washingstation, typically a trough is mounted at a point along the conveyor toprovide a recepticle for wash water applied to the wet web, therebyreducing the level of residual salt in the web. Means for reducing thesalt content of the wet web include applying a continuous ordiscontinuous curtain of water, applying water in a spray pattern,submerging the web in a trough of water, flooding the web and the like,thereby retrieving some salt which has absorbed or adsorbed. Afterwashing, the web is transferred into a means for substantial removal ofwater. Such means include one or more than one single means, forexample, a rotary/thru air dryer, a heated drum dryer, an infraredheating source, hot air blowers, microwave emitting source, and thelike, all which are known and used in web drying processes.

All processing waters except that which is driven off in the dryerexhaust, are captured and recycled to the process; these waters arecollected in what is identified as the "white water" tank. Web basisweight is controlled by regulating the concentration of superabosrbention sensitive polymer and fiber components in the feed or "stock" tankwhich is fed along with the "white water" at prescribed solids contentinto the formation chamber (ie inclined wire machine). After exiting thedryer the web is edge trimmed and rolled onto cores. The water containsa salt. The salt can be sodium chloride, or a alkali (NA, K, Li) oralkaline (Ca, Mg, Ba) carbonate, or an alkali or alkaline sulfate. Thepreferred salt is sodium sulfate because chloride salts contribute tometal stress cracking, and carbonates have the effect of furtherneutralizing the SAP.

The following test methods were used to determine the properties of thesuperabsorbent, ion sensitive polymers and the wet-laid, superabsorbent,ion sensitive polymer particle-impregnated structure where indicated.

Residual Sodium Sulfate salt in web is measurable with a Cole ParmerSulfate Test Kit available from Cole Parmer as Product Code No.05542-23. The procedure consists of extracting the sufate with water anddetermining the sulfate content from the following procedure.

1. An approx. 2 in. diameter disc of the web sample is cut and weighed.The sample is placed in a beaker containing 150 mls. of deionized waterfor about three minutes. The supernatent is then poured off and retainedfor testing as follows:

2. The vial is rinsed with a small portion of the supernatent to betested. The vial is filled with the supernatent to the lower scoreline(150 ml sample).

3. Phenolpthalein indicator is used to check and adjust to neutral pH,e.g. if the sample turns red, hydrochloric acid 10 N is added withswirling until the color is gone.

4. 1(one) dipper (contained in the cap) of sulfate indicator powder isadded to the vial and allowed to dissolve.

5. Isopropyl alcohol is added to raise the liquid to the upper scorelinefollowed by swirling.

6. Standard barium chloride solution provided in the kit is addedcounting dropwise (mixing after each drop) until the yellow color turnsto dull red or orange. The number of drops added is recorded.

7. The number of drops of standard barium chloride consumed ismultiplied by 25 to indicate the mg per liter (ppm) of sulfate presentin the supernatent sample as the following calculation: ##EQU1##Absorbency Under Load (AUL) for Particulate SAP

This test is designed to determine the absorbency under load of aparticulate superabsorbent, ion sensitive polymer. This is a measure ofthe amount of saline (0.9% wt/% NaCl aqueous solution) absorbed by theion sensitive polymer while a predetermined amount of weight is appliedto the ion sensitive polymer gel and indicates the effectiveness of theion sensitive polymer's absorbency in relation to actual use conditions.

Absorbency under load is measured using a plastic petri dish withelevating rods and a 1.241" OD×0.998"lD×1.316" long plexiglass tube witha wire net (100 mesh) at the bottom of the tube. The particle size ofthe test samples is between 30 to 50 mesh, (through 30 and retained on50).

A test sample, 0.160±0.01 g is weighed out and recorded as S₁. Thesample is placed in the plastic tube and is spread evenly over the wirenet. A specified weight(e.g. a 100 g, 200 g or 300 g weight yielding 0.3psi, 0.6 psi and 0.9 psi load, respectively) and a disc are placed onthe sample. The assembly (polymer sample, tube, disc and weight) isweighed and recorded as W₁. The assembly is then placed in a petri dishcontaining 40 ml 0.9% saline aqueous solution. After one hour ofabsorption, the assembly is removed from petri dish and excess salineblotted from the bottom. The assembly is weighed again and this valuerecorded as W₂. Absorbency under load (AUL) is equal to (W₂ -W₁)/S₁ andis express in g/g.

Absorbency Under Load (AUL/(for Web Sample)

This test is designed to determine the absorbency under load of a webcontaining a mixture of superabsorbent polymer and fiberous materials.This is a measure of saline (0.9% wt/% NaCI aqueous) solution absorbedby the web while a predetermined amount of weight is applied to the weband indicates the effectiveness of the web's absorbency in a diapersystem under the weight of a baby. Absorbency under load is measured bycutting a 2 in. diameter circular sample with a die cutter. The sampleis oven dried for 2 hours and then weighed to +/-0.1 grams. Prior totesting the sample is cooled in a controlled environment (70° C., 50%RH). The sample holder is then dried with a hand-held heating blow-dryerto complete dryness. The sample holder has small feet on the bottom toinsure a clearance between the bottom of a saline liquid reservoir andthe holder. The volume of saline solution to be added to the liquidreservoir is determined by adding a measured amount of saline solutionto the reservoir until the liquid level rises to the top of theperforated plate(s) of the sample holder(s). This volume of salinesolution is recorded as X. The volume of the saline to be added to thereservoir is X+120 mls. The circular web sample is placed top side down,inside the holder. The total weight of the sample in it's holder isrecorded as the dry weight. A weight (providing load of 0.5 psi) isplaced on top of the web sample. The reservoir is filled with X+120 mls.of 0.9% saline solution at a temperature of 23+/-1° C. Simultaneouslythe sample holder(s) is placed into the solution. After ten minutes ofswelling, the sample holder(s) are removed from the reservoir andallowed to drip approximately 60 seconds. The weight is removed. Theweight of the wet sample is re-weighed in the sample holder (wetweight). Calculations:

    absorbed weight=(total weight of wet sample and holder)minus (total weight of dry sample and holder)

    AUL (g/g)=absorbed weight divided by oven dried weight of sample

Materials of Web Construction

The fibers used may be filament or staple or a combination of a minoramount of filament and a major amount of staple, or staple fibers ofvarying lengths. The essential fibers in the web are cellulose acetate(CA) and wood pulp. Optional man-made fibers can be included but are notcritical. Polyolefin fibers, polyester fibers and bicomponent fiberscould be included. Preferably, all of the fibers used are CA and Pulpstaple fibers, generally of length from 1 to 100 mm. In a preferredembodiment, a minor amount (about 20%-30% of the fibrous portion) ispolyethylene fiber (type 103 sold under the TREVIRA® trademark), andfrom about 2 to 10% of the fibrous portion is made of bicomponent fiberssold under the type 105 Celbond® trademark of TREVIRA. The staple fibersare preferably of 10 to 50 mm in length. The greater the length, thegreater the strength of the wet web structure up to a point wheregreater fiber length may adversly affect processing of the furnish,material cost, and web uniformity. Cellulose acetate staple is usuallyavailable in lengths of 2 to 50 mm. The more preferred lengths forcellulose acetate are from 0.25 to 0.75 inch (8 to 19 mm), and mostpreferred are lengths of about 0.5 in. (≡12 mm). Cellulose acetatestaple is commercially available from Celanese Acetate, Charlotte, N.C.The denier per filament (dpf) for the cellulose acetate fiber is notcritical. Preferably cellulose acetate having 1.8 dpf and 12 mm length(0.5 inch) is used. Longer lengths could be used but at small denier,fiber entanglement can lead to less uniformity in the web.

Wood pulp fluff of typical length of about 8 mm is used in the wet laidnonvoven industry and is also suitable in the practice of the process.Wood pulp fluff fibers can be obtained from well-known chemicalprocesses such as the kraft and sulfite processes. Suitable startingmaterials for these processes include hardwood and softwood species,such a alder, pine, douglas fir, spruce and hemlock. Wood pulp fiberscan also be obtained from mechanical processes, such as ground wood,refiner mechanical, thermomechanical, chemi-mechanical, andchemi-thermomechanical pulp processes. However, to the extent suchprocesses produce fiber bundles as opposed to individually separatedfibers or individual fibers, they are less preferred. However, treatingfiber bundles is not within the scope of the present disclosure.Recycled or secondary wood pulp fibers and bleached and unbleached woodpulp fibers can also be used. Details of the production of wood pulpfibers are well-known to those skilled in the art. These fibers arecommercially available from sources including Weyerhaeuser Company,Buckeye Cellulose, and Rayonier.

Superabsorbent, ion sensitive polymer, as used herein, refers toparticulate, water insoluble, but water swellable, materials which willpass through a fine mesh screen (U.S. seive). A fine mesh screen means ascreen which allows particles of about 250 micron size or smaller topass through. For example, particles passing through a 65 mesh screencorrespond to particles having a particle size of less than 208 microns.Any metric or equivalent fine mesh screen may be used to give particleshaving this upper limit of particle size. A sizable portion of theparticles used herein pass through a 100 mesh seive and are smaller than150 microns. A minor portion of the particles will pass through even a325 mesh seive which corresponds to 45 microns or less.

The superabsorbent-polymers in particulate form as specified abovegenerally fall into three classes, namely, starch graft crosslinkedcopolymers, crosslinked carboxymethylcellulose derivatives, andhydrophilic polyacrylates. Examples of such absorbent polymers arehydrolyzed starch-acrylonitrile graft copolymer, a neutralizedstarch-acrylic acid graft copolymer, a saponified acrylic acidester-vinyl acetate copolymer, a hydrolyzed acrylonitrile/carboxylatecopolymer or acrylamide copolymer, a partially neutralizedself-crosslinking polyacrylic acid, a partially neutralized, lightlycrosslinked polyacrylic acid polymer, carboxylated cellulose, aneutralized crosslinked isobutylene-maleic anhydride copolymer, and thelike.

The superabsorbent polymer particles need not be but preferably have atleast a portion of their surface which is crosslinked. Surfacecrosslinked poly acrylic acid polymers as taught in U.S. Pat. Nos.4,507,438, 4,541,871, 4,666,983, 5,002,986, 5140,076, 5,164,459,5,229,466, 5,322,896, 5,597,873, and EP 509,708 and falling under thefollowing particle size limitations are suitable herein. The particlesare typically isolated from the surface treatment of primary SAPparticles which have been seived through a selected mesh size screen,e.g., a 65 mesh screen (U.S. seive). The fines represent attritedparticle fragments, and it is expected that only a portion of thesurface of these particles have surface crosslinking. The prevalentsource of fine particle size ion sensitive SAP is from the sifting ofthe primary hydrogel process stream from the solution polymerization ofpartially neutralized polyacrylic acid SAP which has been dried, andchopped into granules. The fine particles are recovered from this mass.The fine particles may be recovered from a subsequent process in whichprimary seived material is treated with crosslinker and seived again.Thus the fine particles used herein may be a mixture of particles whichhave not been surface crosslinked along with particles which have beensifted from a mass of primary particles which have been treated withsurface crosslinker. Commercially available ion sensitive SAP whichpasses through a 100 mesh screen are sold as SANWET® IM-3500F fromClariant Corp., ARIDALL® from Chemdal, Palatine, III. and J550 fromSumitomo Corporation of Japan.

The most preferred, but not essential embodiment of SAP, is recoveredfrom the treatment of primary SAP particles with surface crosslinkedfrom an aqueous crosslinker solution preferably comprising water, a diolselected from a C₃ to C₆ diol and a crosslinking compound. The water anddiol components of the crosslinker solution comprises from about 1.0 toabout 6.0 percent by weight based upon the weight of the treated SAPpolymer, preferably about 1.5 to about 5.5 percent. The crosslinkersolution preferably has a surface tension of less than about 55 dynesper cm. The fine particles are recoverd from this treated mass which hasbeen seived through a screen which retains the primary particles havinggreater than 65 mesh, U.S. seive.

The web structure is formed as a wet-laid nonwoven by any of thetechniques known for wet-laying nonwoven fabrics, for example thosedescribed in "Manual of Nonwovens" by R. Krcma (4th Edition 1974,Textile Trade Press, Manchester) at pages 222 to 226. In general, thefiber and particles are wet-laid in a process similar to a conventionalpapermaking process. The fiber and particles in the aqueous suspensionare continuously deposited on the moving foraminous support. The woodpulp fibers may need to be refined, but this is not essential in thepractice of the invention. It is preferred to mix the superabsorbentpolymer particles into the slurry after refining has been completed.

The furnish can be poured at a controlled rate onto a substantiallyhorizontal mesh screen, or the furnish may be deposited on an inclinedmesh screen travelling upwards through the slurry. Alternatively, thefurnish can be deposited on a mesh screen which is at the surface of asuction drum. The mesh size of the screen should be such as to alloweasy drainage of water but to retain the solids; the most suitable meshsize will generally be in the range 0.2 to 1.5 mm. The mesh can be ofmetal wire or synthetic polymer, for example polyester filament. Thebasis weight of the resulting dried web having no more than 0.5%moisture content is preferably from 100 to 500 g/m² (gsm), morepreferably from 125 to 275 gsm, and most preferably webs of 150 and 250gsm are both utilized in a two layered absorbent component for adisposable diaper.

The fiber/SAP/solids content of the slurry referred to below, wherebysalt is not included in reference to slurry solids, are deposited on theforaminous support (wire) is generally in the range 0.1 to 50 g/litersolids content, preferably 0.2 to 20 g/liter, and more preferably 0.2 to≈5 g/liter. Depending on the feed rate of furnish on the wire and thespeed of the line, a solids content in the area of 0.2 to 2 g/liter canbe run and conditions adjusted so that a basis weight of from 100 to 500gsm can be achieved on typical conventional wet-laying machinery. Aportion of the water content of the slurry is drained from the depositedfiber/SAP layer while it is supported on the mesh screen, preferablywithout the aid of suction applied below the screen. Optionalcompression rolls can be used and may be desired when dryer capacity islimited and particulaly when making higher basis weight webs (350 gsmand above). The solids content of the wet-laid web as it is taken offthe mesh screen is preferably at least 5% and most preferably at least10% by weight, and it is generally not more than 30% and usually notmore than 20% by weight prior to treatment with water.

The formed wet web is then passed under the means for reduction of salt,which is preferably a continuous flow of water in the form of a curtainof flowing water. The delivery rate of the water to the web is adjustedto be sufficient to reduce the residual salt content to the desiredlevel in the web.

The extent of salt removal is directly proportional to the wash waterflow per sq. area of web. Salt content in the web with no washing stepis about 40% when the salt content in the white water is about 4%. Washwater flow rates between 0.5 and 5.0 gal./sq. ft. are preferred. Morepreferably, in the use of a water curtain, the water flow rate should bebetween 0.5 and 2 gal./sq ft. Most preferably the water flow rate shouldbe between 0.75 and 1.25 gal./sq. ft. of web.

The wet web, after water treatment is brought to substantial drynessusing any suitable techniques generally employed in papermakingincluding passage of the web around a heated drum, passage between aseries of heated rolls, or on a flat bed, through air dryer.

The wet-laid nonwoven structure can optionally include dispersedparticles such as silica, a zeolite or a mineral clay, such as kaolin orbentonite. Such particles, which preferably are not used at more than10% by weight of the nonwoven fabric, can be added to the furnish asdescribed in EP-A437816 or incorporated in the superabsorbent particlesas described in WO-A-92/19799.

EXAMPLES Comparative Example

Using a small wet-lay web former available under the Bruderhaus®trademark, the following is made:

Superabsorbent, ion sensitive polymer particles having been sifted andretained on a 65 mesh (U.S. seive) screen, and corresponding toparticles of greater than 208 microns ) were used in this example. Thesolids in the slurry comprised about 2.5 g/l solids, with solidscomprising 60% of the SAP particles and 40% of the fiber portion. Thefiber portion consisted of 100% CA fiber. The Na₂ SO₄ concentration inthe white water was maintained at approximately 4.2%. The furnish waspoured on a moving mesh screen at a flow rate of 2 gpm to form a 12 in.wide moving wet web. The web was advanced at the rate of 0.9 m/min. andwas passed under a curtain of water flow delivered at a rate of 2.67gpm. The resulting web was dried over a rotary through air dryeroperated at 0.9 meter per min. to yield a structure having a basisweight of 152 gsm. The measured residual Na₂ SO₄ salt in the web was34.7%. The AUL for the web was 9.8 g/g, corresponding to AUL per unitSAP of 16.4 g/g.

The dried sheet exhibited a rough texture which can be felt by the skinthrough a thin nonwoven fabric of the type typically used as a topsheetin absorbent personal hygiene articles.

EXAMPLE 1

Particulate SAP ion sensitive polymer particles which pass through a 65mesh (U.S. seive) screen, and corresponding to particles of less than208 microns were used. The solids in the slurry comprised about 2.5 g/lsolids, with solids comprising 60% of the SAP and 40% fiber. The fiberportion consisted of approx. 6.25% of 0.5 in. (1.5 dpf) polyesterfibers, 40.75% of 0.25 in. (1.8 dpf) cellulose acetate fibers and 40.75%of 8 mm wood pulp fibers and 12.5% polyethylene-sheath/polyester-core,concentric bicomponent fibers (Cellbond) The Na₂ SO₄ concentration inthe white water was maintained at approximately 4.2%. The furnish waspoured on a moving mesh screen at a flow rate of 2 gpm to form a 12 in.wide moving wet web. The web was advanced at the rate of 0.9 m/min. andwas passed under a curtain of water flow delivered at a rate of 2.67gpm. The resulting web was dried over a rotary through air dryeroperated at 0.9 meter per min. and at 485° F. to yield a structurehaving a basis weight of 152 gsm. Salt content in the web as determinedby the above method was 30.1%. The AUL for the web was 14.2 g/g,corresponding to AUL per unit SAP of 23.7 g/g. The texture of the webwas unexpectedly much smoother than the comparative example. The textureof a thin topsheet with the web behind it was more pleasing to thetouch. Example 1 exhibited surprisingly better AUL performance versusthe comparative example which was essentially of the same basis weight,percent loading of SAP, and similar level of residual salt in the web.The AUL performance of the web of Example 1 was unexpected, since thefine particle size SAP in that web was not expected to perfom as well asthe conventional SAP having particle size of 200 microns and higher, andcontaining more uniform surface crosslinking.

In view of the salt content in the web of Example 1, reduction in theresidual salt content of the web to 10% to 25%, will result in anincrease in the SAP AUL to greater than 23 g/g.

I claim:
 1. A process for the production of a wet-laid non-wovensuperabsorbent, ion sensitive polymer-impregnated structure, comprisingthe steps of(a) preparing a fiber and water insoluble, water-swellable,superabsorbent, ion sensitive polymer particle (SAP) containing slurry,essentially all of said particles, before use, have a particle size ofless than 250 microns, (b) combining said slurry with a water solutioncontaining a salt to form a furnish, (c) forming a wet web from saidfurnish on a moving foraminous support, (d) washing said wet web in-linewith water, and (e) drying said web, wherein the residual salt contentof the dried web is from less than 40%, and wherein said web has anabsorbency under load (AUL) of greater than or equal to 13 g/g.
 2. Theprocess of claim 1 wherein said superabsorbent, ion sensitive polymer isa crosslinked copolymer of 50 to 99.99% by weight ethylenicallyunsaturated carboxylic monomer and optional copolymerisableethylenically unsaturated monomer.
 3. The process according to claim 1,wherein said furnish has a solids content, excluding dissolved salt offrom 0.1 to 50 grams per liter.
 4. The process according to claim 3,wherein said furnish has a solids content, excluding dissolved salt offrom 0.1 to 20 grams per liter.
 5. The process according to claim 4,wherein said furnish has a solids content, excluding dissolved salt offrom 0.1 to about 5 grams per liter.
 6. The process of claim 1 whereinsaid washing is by a continuous curtain of water at a flow rate of from0.5 to 5 gal./sq. ft of said web.
 7. The process of claim 6 wherein saidwashing is by a continuous curtain of water at a flow rate of from 0.5to 2 gal./sq. ft of said web.
 8. The process of claim 7 wherein saidwashing is by a continuous curtain of water at a flow rate of from 0.75to 1.25 gal./sq. ft of said web.